Molecular beam apparatus of the maser type



Feb. 21, 1961 s. A. JOHNSON EI'AL 2,972,697

MOLECULAR BEAM APPARATUS OF THE MASER TYPE Filed June 26, 1958 THERMO A I80 H5475? REGULATOR A INVENTORS ATTORNEY MOLECULAR BEAM APPARATUS OF THE MASER TYPE Stanley A. Johnson, Brooklyn, N.Y., and Ferris Eugene Alger, New Hope, Pa., assignors to PRD Electronics, Inc., a corporation of New York Filed June 26, 1958, Ser. No. 744,739

11 Claims. or. 313-231 This invention relates to apparatus for producing a beam of molecules which may be used for the amplification or generation of electric waves within the microwave range.

For apparatus of the Maser type generally, see the articles published on pages 1253 and 1264 of volume 99, Physical Review, August 15, 1955. See also the article by I. P. Wittke published on pages 291 to 316 of the Proceedings of I.R.E., volume 45, March 1957.

The present invention is concerned with apparatus in which the molecular beam is formed of molecules of a condensible gas, such as ammonia.

An object of the invention is to provide an arrangement for stabilizing the beam, that is, for maintaining the flux of the beam substantially constant. For this purpose the gas is supplied to the beam-forming nozzle through a very small (capillary) passage of extended length.

A further object of the invention is to devise a system in which the molecular material forming the beam is collected and returned to the beam source without the necessity for removing the material from the system, so that the molecular material remains in a permanently closed system and is used over and over again in re peated cycles.

Due to the recycling of the gaseous material, only one out-gassing is required just before the time the apparatus is initially charged with the gaseous material and after which the system is permanently sealed.

Still another object is to devise a unitary assembly of small weight and capable of being moved from place to place by hand.

A further object of the invention is to devise Maser apparatus producing only a single frequency output. For this purpose, the ammonia gas used to form the beam is compounded from a particular isotope of nitrogen, N The hyperfine structure of this material is symmetrical and there will be no splitting effect. Although the isotope N is very costly, it may be used in the present invention without excessive cost due to the fact that a limited quantity is required and is used over and over again.

A suitable embodiment of the invention is illustrated diagrammatically in the accompanying drawing.

In the drawing, the molecular beam is formed within a closed chamber 1. The gas forming the beam is introduced in the chamber 1 through a conduit 2 which terminates in a beam-forming nozzle 3 provided with a plurality of parallel perforations directed along the axis of chamber 1. The beam from nozzle 3 is directed towards and into a chamber 4 constituting a microwave cavity, one end of the cavity chamber 4 being sealed in the opposite end of the chamber 1 from the nozzle 3. The beam enters cavity 4 through a cut-off tube 5 which prevents loss of wave energy into chamber 1. A waveguide -6 for conducting energy away from the cavity 4 is coupled to the cavity through an iris 4a and is provided with a vacuum-tight wave-transparent window 6a.

2,972,697 Patented Feb. 211 1961 Gas is supplied to the conduit 2 from reservoir 7 which contains the gaseous material in liquid form, such as liquid ammonia, the reservoir 7 being connected to conduit 2 through a beam cut-off valve 8.

For the purpose of stabilizing the flow of the beam, the gas supplied to nozzle 3 from the reservoir 7 is passed through a small passage of extended length embodied in the conduit 2. One suitable construction of such capillary passage is illustrated in the drawing as being formed of a helical groove 9a formed in the outer surface of a plug 9 fitting tightly within the conduit 2, thus providing a helical passage of small transverse dimension between the reservoir 7 and the nozzle 3. By way of example only, the plug 9 fitting within a conduit 2 having an internal bore of A3" diameter would have an outside diameter of and the helical groove would be formed as a 60 groove, 0.003" deep with a pitch of turns per inch. Such a groove has a transverse area of 0.00000503 square inch and a length of about 13 feet. The delivery of such a capillary will follow accurately Poiseuilles law.

For the purpose of removing the ground state molecules from the excited molecules forming the beam, a cage-like assembly of separator electrodes surrounds the beam between the nozzle 3 and the cavity 4. This assembly is of a known type and is formed of a plurality of conducting strips 10 supported at their ends by insulating discs 11 and 12 carried, respectively, by the nozzle assembly and the cavity assembly. Alternate bars 10 are charged positively from a suitable source of high voltage and the remaining bars are charged negatively.

For the purpose of removing the beam gas molecules in chamber 1 after they have performed their function, a cooler 13 in the form of an annular chamber surrounding the separator assembly, is filled with liquefied gas such as air or nitrogen, introduced through an opening 13:: at the upper end of chamber 1. The beam gas is condensed on the surface of cooler 13.

Beam chamber 1 should be maintained at a low gas pressure, of the order of 10' mm. of mercury or lower. Initially, this vacuum is established by the usual pumping operation after which the system is sealed. The low vacuum as far as the beam gas is concerned is maintained by the cooler 13. However, a certain amount of extraneous gases are given off by the walls of chamber 1 and by the elements enclosed therein, which extraneous gases are not condensed on the cooler 13. For the purpose of removing the extraneous gases and maintaining the proper vacuum within chamber 1, a getter chamber 14 is connected to the chamber 1. This getter chambox may be formed of a glass bottle containing a heater filament 15 for evaporating a getter material, such as titanium, from a crucible 16. The ,titanium is deposited upon the inner walls of the chamber 14 as a metallic film 14a and serves as a getter for removing extraneous gases which may be present in the chamber 1. Filament 15 need not operate continuously in the chamber 1 but only at intervals for the purpose of renewing the getter film 14a.

The chamber 14 also functions as an ion trap-by pro viding a separate filament 15a which is energized continuously and operates to ionize gaseous. molecules entering the chamber. These ionized molecules are trapped on the metallic film 14a by maintaining the film at a high negative voltage with respect to filament 15a through a connection 14b leading to a suitable highvoltage source.

The chamber 1 is connected to reservoir 7 through a conduit 7 which by-passes the nozzle conduit 2 and the beam valve 8 and is controlled by by-pass valve 17a.

In order to prevent variations in output of the waye cavity 4 by reason of changes in the dimensions of this cavity due to changes in temperature, the cavity is maintained at a substantially constant temperature by a system formed of a pipe coil 18a surrounding the cavity in good heat transfer relation and having Water pumped therethrough by a pump 19, the water heated by heater 20 and being maintained at a constant temperature by a thermal regulator 21 by regulating either the rate of flow of water or the rate of heating.

During operation, the cooler 13 is filled with liquid gas and maintains the vacuum within chamber 1 by condensing the beam gas after it leaves the beam. During this time the beam valve 8 is open and the by-pass valve 17a is closed. The liquid gas in reservoir 7 is allowed to evaporate by subjecting the reservoir to atmospheric temperature, or to a higher temperature. At this time, the pressure within reservoir 7 will range between 10 atmospheres and atmospheres, depending upon the temperature, and it Will remain at a fixed pressure as long as any liquefied gas remains in the reservoir and so long as the external temperature of the reservoir remains constant. Under these conditions, the beam is formed in chamber 1 by reason of a constant pressure diflerential acting to force gas through the high resistance passage 9a from the reservoir 7 to chamber 1. The high resistance of caprilary passage 9a prevents any substantial change in the flow of gas to nozzle 3 due to changes in ambient temperature with the result that a highly stable beam is formed and is maintained as long as any liquid gas remains in reservoir 7.

Before the liquid gas is completely exhausted in reservoir 7, beam valve 8 is closed and the by-pass valve 17a is opened. Liquid air is removed from cooler 13, and reservoir 7 is immersed in or surrounded with liquid air or liquid nitrogen, so that beam gas evaporated from the cooler 13 passes into the reservoir 7 and is liquefied. For this purpose the reservoir 7 may be placed within a container 18 containing liquid gas. Container 18 may be in the form of a jacket around reservoir 7. After this process continues for a time suflicient to remove substantially all of the ammonia gas from chamber 1, the beam may be re-established in chamber 1 by opening valve 8, closing valve 17a, introducing liquid refrigerant in cooler 13 and removing liquid refrigerant from around reservoir 7. This cycle of operation may be repeated over and over again without loss of beam gas from the system.

It will be seen that the apparatus is of a unitary assembly capable of being moved from place to place. The weight of the assembly may be no more than pounds.

We claim:

1. Molecular-beam apparatus comprising a a beamchamber containing a beam-forming nozzle, a reservoir containing liquefied gas, said reservoir being exposed to a temperature causing evaporation of said liquefied gas Within said reservoir thereby establishing a pressure of several atmospheres within said reservoir, a conduit connecting said nozzle with said reservoir and including a flow-regulating restriction comprising a single capillary passage of relatively small transverse section and long length, and means for maintaining a low vapor pressure within said beam chamber comprising a cooler for condensing upon the surface thereof the gas molecules leaving said beam.

2. Molecular-beam apparatus comprising a bea chamber containing a beam-forming nozzle, a reservoir containing gas under pressure, a conduit connecting said nozzle with said reservoir and including a capillary passage of relatively small transverse section and long length, a cut-01f valve in said conduit between said capillary passage and said reservoir, a by-pass conduit connecting said beam chamber to said reservoir around said capillary passage and said cut-ofi valve, and a by-pass valve in said by-pass-conduit.

3. Molecular beam apparatus comprising a beamchamber containing a beam-forming nozzle, a reservoir containing gas under pressure, a conduit connecting said nozzle with said reservoir and including a capillary passage of relatively small transverse section and long length, a cooler surrounding the beam path within said beamchamber to condense upon the surface thereof gas molecules leaving the beam, and an ion-trap chamber connected with said beam-chamber for removing gases which are not condensed by said cooler.

4. Apparatus according to claim 1 and including means within said ion-trap chamber for forming a film of getter material on the inner wall thereof.

5. Apparatus according to claim 4 wherein said means for forming a film of getter material comprises a heater filament for evaporating titanium within said ion-trap chamber.

6. Molecular-beam apparatus comprising a beamchamber containing a beam-forming nozzle, a reservoir containing gas under pressure, a conduit connecting said nozzle with said reservoir and including a capillary passage of relatively small transverse section and long length, a cooler surrounding the beam-path within said beamchamber to condense upon the surface thereof gas molecules leaving the beam, and a getter chamber connected with said beam-chamber and containing getter material for removing gases which are not condensed by said cooler.

7. Apparatus according to claim 6 wherein said getter material is a film of getter material carried on the inner walls of said getter chamber, and means sealed within said getter chamber for renewing said film from time to time.

8. Apparatus according to claim 7 wherein said means for renewing the film of getter material comprises a bulk supply of getter material sealed Within said getter cham her and heater filament for evaporating getter material from said bulk supply.

9. Molecular beam apparatus comprising a gas-tight system formed of a beam-chamber containing a beamforming nozzle, a reservoir containing gas under pressure, a conduit connecting said reservoir to 'said nozzle and including a capillary passage for restricting the flow of gas to said nozzle, a bypass conduit connecting said beam'chamber with said reservoir around said capillary passage, a by-pass valve in said by-pass conduit, means for maintaining a low vapor pressure within said beam chamber comprising a cooler for condensing upon the surface thereof the beam gas molecules leaving said beam, said cooler being controllable to efiect evaporation vof condensed beam gas from the surface thereof when gas is to be returned from said beam-chamber to said reservoir, and means for cooling said reservoir to effect condensation of beam-gas therein and to efiect removal of beam-gas from said chamber through said by-pass conduit when said by-pass valve is open.

10. Molecular-beam apparatus comprising a beamchamber containing a beam-forming nozzle, a reservoir containing gas under pressure, a conduit connecting said nozzle with said reservoir and including a capillary passage of relatively small transverse section and long length, a by-pass conduit connecting said beam chamber to said reservoir around said capillary passage and a by-pass valve in said by-pass conduit, said by-pass conduit, when said 'valve is open, providing for the unrestricted flow of gas from said chamber to said reservoir.

'11. Molecular-beam. apparatus comprising'an evacuated beam-chamber containing a beam forming nozzle, a reservoir containing liquified gas and being exposed to a temperature below the vaporizing temperature of said liquid gas, whereby the gas exists within said reservoir in gaseous and liquid forms in equilibrium at a fixed pressure equal to its characteristic vapor pressure at the temperature of said reservoir, the said fixed pressure remaining constantas long as gas in liquidrphase remains in the reservoir even if gas is withdrawn from the reservoir, a channel connecting said reservoir to said nozzle to conduct gas from said reservoir to said beamchamber, said channel including a capillary passage of relatively narrow cross-section and great length, the proportions of said capillary passage being such as to reduce the said fixed pressure of the reservoir to a relatively much lower fixed pressure suitable for beam-formation in the said evacuated chamber, whereby a constant and stabilized molecular beam is maintained with in said chamber, a separator structure of cage-like form surrounding the beam from said nozzle and operating when electrically charged to separate certain gas molecules from the beam, and a cooler having a condensing surface surrounding said cage-like separator to intercept and condense thereon gas molecules leaving said beam and passing through said separator.

References Cited in the file of this patent UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFICATE CRRCTIQ Patent No., 2,972,697 February 21 1961 Stanley A Johnson et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line l1 for the claim reference numeral "1'" read 3 Signed and sealed this 7th day of November 1961o (SEAL) Attest:

ERNEST W. SW'IDER DAVID L. LADD Attesting Officer Commissioner of Patents USCOMM-DC ent requiring correction and UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2,972 697 February 21, 1961 Stanley A, Johnson et a1.

It is hereby certified that error appears in the above numbered petthat the said Letters Patent should read as corrected below.

Column 4, line ll for the claim reference numeral "1'" read 3 Signed and sealed this 7th day of November 1961.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer DAVID L. LADD Commissioner of Patents USCOMM-DC 

